Visible light-driven photophysics and photochemistry of water-soluble metalloporphyrins

Metal ions can form normal (in-plane) metalloporphyrins, fitting into the central hole of the porphyrin ring, or several of them are located out of the ligand plane, resulting in sitting-atop (SAT) complexes. Kinetically inert water-soluble complexes of Mn(III), Co(III), and Ni(II) with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin display a weak, short-lived fluorescence. This can be affected by elongation of the alkyl substituent and using micellar environment in the case of Mn(III) porphyrins. In the presence of a suitable electron donor (triethanolamine, TEOA) and acceptor (methylviologen, MV2+), these metalloporphyrins proved to be efficient photocatalysts transferring electrons between the ground state donor and acceptor via outer-sphere mechanism. In these systems triplet excited-state Mn(III) and Co(Ill) porphyrins are dynamically quenched with TEOA. The Mn(II) and Co(II) complexes formed in this way need also photoexcitation for the transfer of electron to the ground-state acceptor. However, the triplet excited state of Ni(II)TMPyP4+ cannot be dynamically quenched with TEOA. Instead, this electron donor forms an associate with Ni(II)TMPyP4+ in a ground-state equilibrium. The lifetime of the triplet excited state of this species is much longer than that of the nickel(II) porphyrin alone, and it can undergo an efficient dynamic oxidative quenching with MV2+. Thus, a one-step electron transfer can be realized between the electron donor and acceptor, generating MV center dot+, which can be utilized for hydrogen generation from water. Lanthanide(III) porphyrins are of typical SAT complexes, the photophysical and chemical features of which can be tuned by the size of the metal center. Anionic, early lanthanide(III) mono- and bisporphyrin complexes exhibit very similar photoinduced properties as a consequence of a special type of aggregation, through the peripheral substituents. The rather slow formation of complexes and transformation between the mono- and bisporphyrins can be accelerated by the irradiation of the system. These by-processes play considerable roles beside the photoredox degradation and demetalation. Depending on the wavelength of irradiation, two types of photoproducts can appear: during the photolysis at the Soret-band, a radical type intermediate can be observed, which disappears in dark. However, irradiation at the Q-bands, generates the formation of a new, stable photoproduct. (C) 2016 Elsevier B.V. All rights reserved.